In recent years, preservation and stabilization of color image quality and uniformity of the color image quality among a plurality of individual color image forming apparatuses are increasingly-demanded for color image forming apparatuses. The stability of color density is particularly desired, without being affected by an installation environment, deterioration with age and differences between apparatuses. Hereinafter, differences among apparatuses are referred as individual differences.
Conventionally, as a color density sensor to detect the color density, there is a senor that light beams from a light source is radiated to a specific reflection area of a surface of a measured object such as a sheet, and the light beams reflected within a predetermined angle range, among the light beams reflected by the reflection area, enter to a photoelectric conversion element through a lens. For example, Patent Document 1: unexamined Japanese patent application publication No. H10-175330 discloses the above color sensor. From an output of the photoelectron conversion element by receiving the light, a color density of a toner image for density detection formed by toner of each color is measured. Meanwhile, the above color density sensor enables accurate detection by arranging the light source, the lens and the measured object in predetermined optical position relations, though the measured object is displaced slightly in a direction perpendicular to a conveyance direction of the measured object.
As a calibration method of color density measured by the above color density sensor, there is a method that reflection ratios of a calibration plate and a calibration comparison plate are obtained, and a rate in respect to reference white color such as white color of a sheet used in an actual apparatus is stored so as to perform correction of the individual difference as an initial calibration. Namely, in the above calibration method, by defining white color reference using white color of the sheet, another white color reference plate is not use. For example, there is a description related to the above color density calibration method in Patent Document 2: unexamined Japanese patent application publication No. 2001-86297.
Also, Patent Document 3: unexamined Japanese patent application publication No. 2006-235490 discloses a color image forming apparatus omitting the white color reference plate to cope with a problem that the white color plate is tainted by paper powder and toner.
Further, as another calibration method using a while portion of a transfer material as the white color reference plate, to eliminate effects of variation of the white portion of the sheet, there is know a method to correct a sensor output in accordance with shading of the toner when the toner density is low. For example, the description related to a correction method of the sensor output is in Patent Document 4: unexamined Japanese patent application publication No. 2003-149903.
Patent Document 1: unexamined Japanese patent application publication No. H10-175330
Patent Document 2: unexamined Japanese patent application publication No. 2001-86297
Patent Document 3: unexamined Japanese patent application publication No. 2006-235490 is omitted
Patent Document 4: unexamined Japanese patent application publication No. 2003-149903.
In the each one of the above convention technologies, the color sensor had problems such that there is occurred deterioration with age by being exposed in vapor of oil and wax from the sheet, and the light receiving element is affected by heat radiated from the sheet resulting in an adverse effect on an output characteristic. Namely, in the image forming apparatus employing a thermal compression fixing method to fix an image, if the color density sensor is disposed at a downstream side of the fixing section to perform a fixing process, the color density sensor tends to be subject to the deterioration with age and the adverse affect on output characteristic as described above.
An object of the present invention is to enhance detection accuracy of the color density sensor equipped in the color image forming apparatus by suppressing a thermal effect from the sheet having passed through the fixing section to perform the fixing process and taint caused by being exposed in the vapor of oil or wax. Also, by enhancing the detection accuracy, the other object of the present invention is to provide the color image forming apparatus having a high stability in the color density of a formed image, without being affected by the installation environment, the deterioration with age and the individual difference and a control method of the apparatus thereof.